Move prompt caching setting to Application Settings JSON column

Note

@missy-gitlab Summarizing from Slack: "Cascading settings predates the JSON column recommendation" so it might work, however "it would be the first time trying it" and this is something time sensitive

@praba.m7n @missy-gitlab - Thanks for raising this, would have been great to have this and other related attrs in a single jsonb within application_settings, for all the reason mentioned in the doc. Loading the column cache for application_settings is one of the most expensive queries because of how wide it became and it's called more frequently

Application Settings have moved to being stored in a JSON column instead of individual columns. However, this hasn't been tried with Cascading Settings.

Implementation

1. Try to update the cascading setting model_prompt_cache_enabled to be added to the code creation application setting JSON column instead
2. If there are issues with Cascading Settings in a JSON column, address those
3. Migrate the existing column setting over to JSON once it's verified it's working